A. Field of the Invention
The invention relates to a composition suitable for use as a chemical wash in well cementing procedures. It also relates to a method for using the chemical wash composition.
B. Description of the Prior Art
Rotary drilling techniques are commonly used for drilling wells into the earth. In the rotary drilling of a well, a drilling fluid or mud is circulated between the surface of the earth and the bottom of the well. Drilling muds which are commonly used include water based drilling muds, including both clay and polymer containing muds, oil based drilling muds, and emulsions. Drilling muds serve many useful purposes including the removal of earth cuttings from the well, control of formation pressures, and cooling and lubrication of the drill bit used in forming the well. However, there are also certain detrimental characteristics associated with drilling muds.
Among the problems associated with drilling muds, is that the liquid phase of a drilling mud tends to flow from the well into exposed permeable formations with the result that mud solids are filtered out on the wall of the well and a filter cake is formed thereon. Mud filter cakes are detrimental in the completion of wells in that they interfere with obtaining a good cement bond between the wall of the well and the conduit, or casing, positioned in the well. Also, drilling muds frequently contain components which are incompatible with fluids which one may desire to inject into a well containing a mud. For example, it has long been recognized that if certain cement slurries containing free polyvalent metal cations, especially calcium, are brought into contact with muds containing clay or certain polymers, a very viscous and detrimental plug can form in the vicinity of the mud-cement interface. High pressures required to move such a plug can rupture tubing, or make it necessary to stop pumping to avoid rupturing the tubing with the result that appreciable volumes of cement are left inside the tubing. Also, the high pressures can cause fracturing of the formation, thus causing loss of cement to the formation and formation damage. Another example of mud-cement incompatibility is that lignins, frequenty used as dispersants in high density muds, can cause excessive retardation in cements if the cement becomes comingled with the mud.
For these reasons, various techniques have been developed for the removal of drilling muds from a borehole, particularly in the context of injecting a fluid into the borehole where said fluid is not compatible with the mud, and even more particularly, in the context of cementing. A common technique is to employ a "spacer" or "chemical wash". Although it is not always clear in the literature whether a particular fluid is a spacer or a chemical wash, a spacer is generally characterized as a thickened composition which functions primarily as a fluid piston in displacing the mud. Frequently, spacers contain appreciable quantities of weighting materials and also include fluid loss control agents. Chemical washes, on the other hand, are generally thin fluids which are effective principally as a result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because chemical washes are generally too thin to have good solids carrying capacity.
Chemical washes have also been employed to precondition air drilled boreholes in preparation for cementing.
Oil based chemical washes or spacers are disclosed in Means et al., U.S. Pat. No. 3,086,938; Messenger, U.S. Pat. No. 3,688,845; Hill et al., U.S. Pat. No. 3,749,173; and Motley et al., U.S. Pat. No. 3,820,602 and U.S. Pat. No. 3,849,316. Oil based chemical washes or spacers, however, generally do not perform well with water based muds.
Emulsion spacers or washes are disclosed in Parker, U.S. Pat. No. 3,625,286; Tuttle, U.S. Pat. No. 3,653,441; Carney, U.S. Pat. No. 3,850,248; and in Journal of Petroleum Technology, August, 1974, page 856. Webb, U.S. Pat. No. 3,532,168 teaches a micellar solution. Emulsions, however, do not lend themselves to easy preparation in the field.
Substantially water based fluids have also been proposed for use as spacers or chemical washes. A water buffer fluid is taught in Neft. Khoz., v. 44, n. 12, p. 25-29 (December 1966) (in Russian--English abstract at 1967 Petrol. Abstr. 80611). Williams, U.S. Pat. No. 2,848,051, employed a composition consisting essentially of insoluble inorganic materials in oil or in water. In U.S. Pat. No. 3,299,952, Savins teaches use of an aqueous solution of polyvinyl alcohol and borate ions. Wieland et al., U.S. Pat. No. 3,878,895, teaches an aqueous mixture of guar gum, a clay stabilizer such as an alkali metal chloride, calcium chloride, or zirconium oxide, a turbulence inducer such as the condensation product of mononaphthalene sulfonic acid and formaldehyde, an acid-soluble particulate such as powdered limestone, and polyethyleneimine. While some of these systems have found some measure of commercial success, most have suffered from at least one deficiency, such as compatibility with only a rather limited number of muds, a tendency to flocculate when used with certain cements, difficulty in mixing, susceptibility to bacterial degradation, difficulty in cleaning of equipment, and waste disposal problems. Accordingly, the industry has continued to search for new and improved spacer and wash compositions.
Other relevant art includes Messenger, U.S. Pat. No. 3,467,193, which taught use of an aqueous preflush containing a dispersant, which was followed by a spearhead of cement slurry containing a turbulence inducer, which was followed in turn by a densified cement slurry. Messenger, U.S. Pat. No. 3,884,302, used an aqueous alkaline flush having a pH of at least 10, which was followed by an aqueous fluid which formed a viscous film at the interface with the alkaline flush. Messenger, U.S. Pat. No. 3,348,612, further illustrates the state of the art. Crinkelmeyer, U.S. Pat. No. 3,605,897, teaches that glycols can be used to separate slugs of fluids. A surfactant is taught in J. Jap. Ass. Petrol. Technol. v. 35, No. 2, pp. 83-88, March 1970 (in Japanese; English abstract at Petrol. Abstr. v. 11, part 2, entry 143,459 (Apr. 17, 1971)). Various surfactant systems are reviewed at Neft. Khoz. No. 12, pp. 9-12, December, 1974 (in Russian; English abstract at Petrol. Abstr. v. 15, part 2, entry 204,636 (1975)).
Other chemical washes in commercial use include aqueous solutions of phosphates, alkaline earth metal lignosulfonates, ferrochrome lignosulfonates, tannins, fatty amides, fatty esters, alcohols, and the like, and various mixtures thereof. See "Spacers and Washes for Dowell Cementing Services", form DWL 1451-4M-674 published by The Dowell Division of The Dow Chemical Company. Caustic and acids are sometimes used ahead of cement, but for their pH or reactive properties, rather than as surfactants.
Although it is conventional to use fluid loss control agents in spacer compositions, it has heretofore not been possible to impart fluid loss control to chemical washes. Because of the relatively non-viscous nature of chemical washes, attempts to carry a sufficient load of conventional fluid loss control materials in the wash have been unsuccessful. Also, most fluid loss control materials are effective only when the fluid is at least somewhat viscous, yet increased viscosity generally results in a less effective chemical wash since an increase in viscosity results in a loss of turbulence in the borehole.